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Atomistically informed phase field simulations for grain growth and phase transformation Suhane, Ayush


The mechanical properties of metals and alloys are extremely sensitive to the microstructure. Different metallurgical processes, such as recrystallization, grain growth, and phase transformations, may modify the microstructure, where each process proceeds by the migration of interfaces that may be strongly affected by the presence of solutes and/or impurities due to solute drag. Quantifying the solute drag requires expensive and time-consuming experimental trials, which are further limited due to the vastly different length scales of solute segregation (few nm) and microstructural features such as grain sizes (few µm). This study presents a computational approach that integrates the microstructure evolution model, i.e., here the phase field method, with atomistic simulations, i.e., density functional theory simulations (DFT), to identify the role of solutes on microstructural processes. First, the experimental migration rates of a single well-defined grain boundary (GB) in Au during recrystallization heat treatments are rationalized using DFT calculations in combination with a continuum solute drag model. Here, an approach to determine the effective segregation energy from atomistic calculations is proposed, suggesting strong solute drag due to 2 ppm Bi impurities in the Au sample. In the microstructural scale, different grain boundaries exist with variability in GB properties, such as GB mobility and solute drag. A phase field model with a friction pressure is used to simulate solute drag on individual GBs. The simulations considering the variability in GB properties indicate that a representative GB can be defined that mimics the average grain size evolution in the presence and absence of solutes. Using the solute binding energies for five solutes in nine different grain boundaries in FCC-Fe, the anisotropic phase field simulations suggest a minor role of segregation anisotropy on austenite grain growth, and as a result, the Σ5(310)[001] GB is considered as the representative GB. A solute trend parameter is proposed to identify solutes that promote grain refinement in agreement with experimental observations. Finally, the atomistically informed approach is extended to phase transformation in binary alloys. Here, phase field simulations that explicitly considered solute segregation in nanocrystalline materials agree with the steady-state solute drag model.

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